This invention relates to a semiconductor structure for use in the near infrared region, preferably in the wavelength range from 1.3 to 1.6 μm, and to a method of manufacturing a semiconductor structure of the aforementioned kind.
In the area of computers, telecommunications and fiber optics there is a need for optical circuits working in the near infrared region, i.e., in the range of wavelengths from 1.3 μm to 1.6 μm. Semiconductor devices operating in this wavelength region are generally known. Known emitters and detectors for use in this wavelength region typically comprise heterostructures made from III–V compound semiconductor based materials, for example GaAs, AlGaAs, or InGaAs. These heterostructures are of semiconductor type L i.e. they are characterized by a direct transition of the charge carriers from the conduction band CB to the valence band VB. The direct interband transition is indicated by the arrow in FIG. 1 which shows the energy band structure in momentum space of a type I semiconductor. Because of this direct interband transition, the recombination efficiency of carriers and, hence, the photoluminescent intensity are very high and the carrier lifetime typically is less than a few microseconds.
However, III–V heterostructure technology is very costly. Moreover, hazardous source materials are used in the production of such III–V heterostructures, for instance, if a metal oxide chemical vapour deposition (MOCVD) technique is used. It is therefore desirable to have emitters, for example light emitting diodes, and detectors based on silicon which is approximately two hundred times less expensive than III–V semiconductor material. Further, the integration of Si based emitters and detectors would allow the realization of efficient interconnects between Si-based integrated circuits and internal light sources. Thus optical communications would be possible between components of computers and telecommunications equipment avoiding heat dissipation problems associated with existing circuits.
In the case of infrared emitting circuits there is, however, a general problem. Silicon is namely a type II semiconductor, i.e. it is characterized by an indirect fundamental band gap between the conduction band EC and the valence band EV as can be seen from the energy band structure in momentum space which is shown in FIG. 2. The maximum of the valence band VB and the minimum of the conduction band CB are not directly opposite to each other, but rather a global minimum of the CB is at a value of wave vector k≠0. Due to the principle of conservation of momentum, an electron can recombine with a hole and emit a photon only through exchanging momentum with a third particle, such as a phonon. This recombination process is very rare in comparison with direct transitions in type I semiconductors. Therefore, the recombination efficiency of carriers and, thus, the photoluminescent intensity of Si based emitters is strongly reduced.
Many attempts have been made to develop new concepts of light emitting structures or detectors which can be incorporated in silicon technology. For example, infrared detectors basing on germanium on Si wafers have been investigated, see for example L. Colace et al., Appl. Phys. Lett. 76, 1231 (2000). Moreover, porous silicon has been analyzed (A. G. Cullis et al., J. Appl. Phys. 83, 909 (1997)). In addition, Si-Ge quantum well structures have been investigated (H. Presting et al., Appl. Phys. Lett. 69, 2376 (1996)), as well as quantum dot structures of SiGe (P. Schittenhelm, “Selbstorganisation und Selbstordnung in Si/SiGe-Heterostrukturen”, in “Selected Topics of Semiconductor Physics”, Eds: G. Abstreiter, M. Stutzmann, P. Vogl, TU München 1997, ISBN 3-932749-02-2). Moreover, carbon doped SiGe has been investigated, see T. Brunhes et al., Appl. Phys. Lett. 77, 1822 (2000) and K. Eberl et al., Thin Solid Films 294, 98 (1997). Furthermore, doping of silicon with centers for luminescence, for instance doping with erbium F. Priolo et al., “Excitation and nonradiative deexcitation process of Er3+ in crystalline Si”, Phys. Rev. B 57,4443 (1998), and silicon nanocrystals have been investigated.
However, for reasons such as low efficiency and operation only at low temperatures, none of the above mentioned systems has yet lead to a commercial product. It is only low dimensional semiconductor structures, in particular quantum dots (QD), which have attracted increasing interest from the point of view of fundamental physics and device application. For example, the strained SiGe/Si system has been subject of numerous investigations (O. G. Schmidt and K. Eberi, Phys. Rev. B61, 13721 (2000) an M. Goryll et al., Thin Solid Films 336, 244 (1998)). Optical properties of Ge islands have been widely studied and the complex transition and recombination phenomena in multi-layer structures have been analyzed. Photoluminescence of Si/Ge islands is generally obtained at low temperatures. Recently, some papers reported on room temperature photoluminescence originating from Si/Ge quantum dot structures (H. Sunamura et al., J. Cryst. Growth 157, 265 (1995) and O. G. Schmidt et al., Appl. Phys. Lett. 77, 2509 (2000)). However, no detailed investigations on the optical properties were presented.